As the most popular process for obtaining multicolor images, silver salt photography has so far been widely used in practice. However, silver salt photography has defects in that it requires dark-room working and in that processing with a developer is required after imagewise exposure.
In order to remove these defects, JP-A-59-48764 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application"), etc. describe dry-process silver salt photographic light-sensitive materials, British Patent 249530, U.S. Pat. Nos. 2,020,775, 2,004,625, 2,217,544, 2,255,463, 2,699,394, etc. describe dye diffusion transfer process photographic light-sensitive materials, and U.S. Pat. No. 2,844,574, etc. describe silver dye bleach process photographic light-sensitive materials.
On the other hand, as recording materials not using silver salts, electrophotographic systems in which an apparatus equipped with a multicolor recording mechanism, a heat-sensitive transfer system, an ink-jet system, and the like is employed. However, these have defects in that large-sized equipment is required, that recording reliability is insufficient, and that exchange of expendables is troublesome.
Recently, thermal recording has been noted as a simple and maintenance-free recording system, and multicolor recording using heat-sensitive recording material is being studied. For example, JP-B-51-19989 (the term "JP-B" as used herein means an "examined Japanese patent publication") (corresponding to British Patent 1409831), JP-B-52-11231, JP-A-54-88135, JP-A-55-133991, JP-A-55-133992, etc. describe multicolor heat-sensitive recording materials which, however, use a plurality of heat-sensitive recording materials having different coloration temperatures and therefore provide colors by color mixing, which has disadvantages. In order to overcome this defect, there is a process using a decolorizing mechanism. A decolorizing agent acts on a color-forming unit capable of forming color at a lower temperature simultaneously when a color-forming unit having a higher heat response temperature forms color, as described in, for example, JP-B-50-17868 (corresponding to U.S. Pat. No. 3,843,384), JP-B-51-5791, JP-B-57-14318 and JP-B-57-14319. However, this process has a defect in that a hue of color which is to be formed at a lower temperature is undesirably formed also at the edge of an image which is formed at a higher temperature, thus it is not suited for multicolor recording.
As multicolor recording materials overcoming the above-described defects, recording materials wherein heat-sensitive coloration system, light-sensitive coloration system, etc. are combined are described in Japanese Patent Application No. 61-80787, JP-A-63-172681, JP-A-63-45084 (corresponding to British Patent 8807887), JP-A-63-134282, etc. They comprise a transparent support having provided on one side thereof at least one color-forming unit layer and on the other side at least one color-forming unit layer capable of forming a color of different hue from that of the above-described unit layer, said color-forming units containing a combination of a diazo compound and a coupler as a color-forming component and/or a combination of a leuco dye and a color-developer as a color-forming component.
However, these recording materials still involve many unsatisfactory points in that transparent supports are necessary, and that, since diazo compounds are used, they have a short usable period (life) after their production.